The present invention relates to compounds having antagonistic activity on adenosine A2B receptors. Such compounds are useful in medicaments for treating diseases responsive to reduced levels of A2B receptors as well as use as radioligands for studying biological activity associated with the adenosine A2B receptor.
Adenosine (Ado) is an autocoid (or local hormone) that modulates numerous functions in the cardiovascular and other organ systems. The actions of Ado are mediated by at least four subtypes of cell surface receptors called A1, A2A, A2B, and A3. Because the ubiquity of adenosine receptors (AdoRs) throughout the body of a human, their indiscriminate activation may cause undesirable side effects. Therefore, new drug design approaches to achieve receptor and organ selectivity are needed.
Recently significant advancement has been made in the understanding of the molecular pharmacology and physiology of A2B adenosine receptors. However, due to the lack of highly potent and selective ligands for this receptor subtype, many questions about the patho-physiological role of A2B receptors have not yet been answered (Feoktistov and Biaggioni, 1997; Feoktistov and Biaggioni, 1998). A2B receptors have been implicated in the regulation of mast cell secretion (Feoktistov and Biaggioni 1995), gene expression (Boyle et al. 1996), cell growth (Dubey et al., 1996), intestinal functions (Murthy et al., 1995), neurosecretion (Mateo et al., 1995), vascular tone (Haynes et al., 1995) and asthma (Feoktistov et al., 1998).
U.S. Pat. No. 6,117,878 to Linden discloses the use of 8-phenyl substituted xanthines for the treatment of diseases induced by activation of the adenosine A2B receptor and mast cell activation. These disease states are disclosed as including asthma, myocardial reperfusion injury, allergic reactions including rhinitis, poison ivy induced responses, urticaria, scleroderma arthritis, other autoimmune diseases and inflammatory bowel diseases. In general, antagonists of the A2B adenosine receptor subtype are disclosed to have anti-inflammatory action. U.S. Pat. No. 6,117,878 to Linden is incorporated by reference.
U.S. Patent Application 2002/0002142 to Belardinelli et al. discloses the use of A2B adenosine receptor antagonist compounds for inhibiting mammalian cell proliferation in cells that express the A2B adenosine receptor including human retinal endothelial cells (HREC). Belardinelli discloses such treatment for ischemic injury to retinal vessels, for example, microvascular abnormalities of the retina, retinopathy, prematurity, macular degeneration, and diabetic retinopathy. U.S. Patent Application 2002/0002142 to Belardinelli et al. is incorporated by reference.
The use of A2B antagonists as antiasthmatic agents is supported by the experimental observation that theophylline and enprofylline are used as therapeutic agents (Feoktistov and Biaggioni 1997; Feoktistov et al., 1998). Theophylline is an alkyl-xanthine that is a weak nonselective adenosine antagonist (See Linden et al., Cardiovascular Biology of Purines, eds. G. Burnstock, et al., 1998, pp. 1-20.) However, its use is associated with unpleasant side effects, such as insomnia and diuresis. (See Vassallo et al., Mayo. Clin. Proc. 1998, 73, 346-354). In recent years, the use of theophylline as a bronchodilator, for relief of asthma, has been supplanted by drugs of other classes, i.e., selective β2-adrenergic agonists, corticosteroids, and recently leukotriene antagonists. (See Drazen et al., New Eng. J. Med. 1999, 340, 197-206.). These compounds also have limitations, thus, the development of a theophylline-like drug with reduced side effects is still desirable.
It has been recognized that theophylline and its closely related analogue caffeine block endogenous adenosine acting as a local modulator of adenosine receptors in the brain and other organs at therapeutically useful doses. (See Fredholm et al., Pharmacol. Rev. 1999, 51, 83-133.). In comparison to the other known actions of theophylline, e.g., inhibition of phosphodiesterases, theophylline is more potent in antagonism of adenosine receptors.
As noted the xanthine derivative, enprofylline, is also used to treat asthma. Enprofylline has been reported to block A2B adenosine receptors. However, this compounds only weakly blocks A1, A2A and A3 adenosine receptors.
It has been reported that therapeutic concentrations of theophylline or enprofylline block human A2B receptors, and it has been proposed that antagonists selective for this subtype may have potential use as antiasthmatic agents. (See Feoktistov et al., Pharmacol. Rev. 1997, 49, 381-402; and Robeva et al., Drug Dev. Res. 1996, 39, 243-252. Enprofylline has a reported Ki value of 7 μM and is somewhat selective in binding to human A2B adenosine receptors. (See Robeva et al., Drug Dev. Res. 1996, 39, 243-252 and Linden et al., Vol. Pharmacol. 1999, 56, 705-713.)
Adenosine A2B receptors are expressed in some mast cells, such as the BR line of canine mastocytoma cells, which appear to be responsible for triggering acute Ca2+ mobilization and degranulation. (See Auchampach et al., Mol. Pharmacol. 1997, 52, 846-S60 and Forsyth et al., Inflamm. Res. 1999, 48, 301-307.) Adenosine A2B receptors also trigger Ca2+ mobilization, and participate in a delayed IL8 release from human HMC-1 mast cells. Other functions associated with the A2B AR are the control of cell growth and gene expression, (See Neary et al., Trends Neurosci. 1996, 19, 13-18.) endothelial-dependent vasodilation (See Martin et al., J Pharmacol. Exp. Ther. 1993, 265, 248-2,53.), and fluid secretion from intestinal epithelia. (See Strohmeier, et al., J Biol. Chem. 1995, 270, 2387-2394.) Adenosine acting through A2B receptor subtype has also been reported to stimulate chloride permeability in cells expressing the cystic fibrosis transport regulator. (See Clancy et al., Am. J Physiol. 1999, 276, C361-C369.)
Both of these xanthine derivatives, enprofylline and theophylline, are proven to be effective but with low potency and selectivity at the A2B adenosine receptor subtype (theophylline A2B binding affinity Ki=13 μM; enprofylline A2B binding affinity Ki=7 μM).
Asthma is a complex disease involving the concerted actions of multiple inflammatory and immune cells, spasmogens, inflammatory mediators, cytokines and growth factors. Theophylline has significant side effects that may be related to its A1 receptor antagonism. It is therefore believed that more potent and selective A2B receptor antagonists will provide enhanced asthma treatment.
The A1, A2A and A3 adenosine receptors have been pharmacologically characterized through the use of highly potent and selective agonists and/or antagonists. In contrast the study of A2B receptor has been precluded due to the lack of selective ligands. Researchers such as Jacobson and his coworkers have proposed using the radioligand of 7-amino-2-(2-4-furyl)-5-[2-(4-hydroxy-phenyl)ethyl]-amino[1,2,4]-triazolo-[1,5-a][1,3,5]-triazine ([3H]-ZM241385) as useful radioligand for studying the A2B adenosine receptor subtype (Ji et al., 1999) even though [3H]-ZM241385 has a KD value of 34 nM.
Jacobson and coworkers have also reported some xanthine derivatives endowed with good affinity to the adenosine receptors but without significant selectivity for the human A2B adenosine receptor subtype (Kim et al., 1999; Jacobson et al., 1999). Some non-xanthine derivatives closely related to ZM 241385 have also been synthesized, but, while some of the reported compounds displayed significant affinity to A2B receptors, none of them possessed relevant selectivity versus the other receptor subtypes A1, A2A and A3 (De Zwart et al., 1999).
Similarly, Jacobson and coworkers, with the aim of obtaining non-xanthine antagonists for A2B receptors, modified the structure of 5-amino-9-chloro-2-(2-furanyl)[1,2,4]triazolo-[1,5-c]-quinazoline (CGS 15943), a non selective adenosine receptor antagonist, which appeared to be a suitable starting compound for adenosine receptor antagonists by appropriate substitutions at different positions. In particular an improvement of affinity to A2B receptors was observed when aminoacidic chains are appended to the amino group of position 5 (Kim et al., 1998).
U.S. Pat. No. 5,935,964 to Baraldi et al. discloses triazolo pyrimidines have antagonist affinity for the A2A receptor. Using these compounds as a starting point, Baraldi et al. investigated a series of pyrazolo[4,3-e]1,2,4-triazolo-[1,5-c]pyrimidine compounds for use as A2B receptor antagonists. In particular the compounds with the free amino group at the 5-position, and for example, a phenylethyl chain at the N8 pyrazole nitrogen show good affinity to A2B adenosine receptors. However, none of these compounds demonstrated good selectivity (Baraldi et al., 2001).
Linden, Jacobson and coworkers have also reported a series of anilide derivatives of 8-phenyl-xanthine carboxylic congeners that proved to be potent and selective A2B antagonists (Kim et al., 2000; Ji et al., 2001) and S-substituted 1,3,7-trialkyl xanthine derivatives (U.S. Pat. No. 5,861,405 to Jacobson et al.) While many of the Linden and Jacobson compounds have high adenosine A2B potency, selectivity against other human adenosine receptors are in most cases limited (PCT patent application WO 00/73307 by Linden, Jacobson et al., 2000).
We have now discovered additional novel xanthine compounds having high antagonist affinity for the A2B receptor with varying levels of affinity and superior selectivity compared to compounds previously reported. The above noted references are incorporated herein by reference.